kmsan: fix memcpy tests

Recent Clang changes may cause it to delete calls of memcpy(), if the
source is an uninitialized volatile local.  This happens because passing a
pointer to a volatile local into memcpy() discards the volatile qualifier,
giving the compiler a free hand to optimize the memcpy() call away.

Use OPTIMIZER_HIDE_VAR() to hide the uninitialized var from the too-smart
compiler.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Alexander Potapenko <[email protected]>
Suggested-by: Marco Elver <[email protected]>
Reviewed-by: Marco Elver <[email protected]>
Cc: Dmitry Vyukov <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

mm/kmsan/kmsan_test.c

@@ -419,6 +419,7 @@ static void test_memcpy_aligned_to_aligned(struct kunit *test)
  kunit_info(
  test,
  "memcpy()ing aligned uninit src to aligned dst (UMR report)\n");
+ OPTIMIZER_HIDE_VAR(uninit_src);
  memcpy((void *)&dst, (void *)&uninit_src, sizeof(uninit_src));
  kmsan_check_memory((void *)&dst, sizeof(dst));
  KUNIT_EXPECT_TRUE(test, report_matches(&expect));
@@ -441,6 +442,7 @@ static void test_memcpy_aligned_to_unaligned(struct kunit *test)
  kunit_info(
  test,
  "memcpy()ing aligned uninit src to unaligned dst (UMR report)\n");
+ OPTIMIZER_HIDE_VAR(uninit_src);
  memcpy((void *)&dst[1], (void *)&uninit_src, sizeof(uninit_src));
  kmsan_check_memory((void *)dst, 4);
  KUNIT_EXPECT_TRUE(test, report_matches(&expect));
@@ -464,6 +466,7 @@ static void test_memcpy_aligned_to_unaligned2(struct kunit *test)
  kunit_info(
  test,
  "memcpy()ing aligned uninit src to unaligned dst - part 2 (UMR report)\n");
+ OPTIMIZER_HIDE_VAR(uninit_src);
  memcpy((void *)&dst[1], (void *)&uninit_src, sizeof(uninit_src));
  kmsan_check_memory((void *)&dst[4], sizeof(uninit_src));
  KUNIT_EXPECT_TRUE(test, report_matches(&expect));